1. Field of the Invention:
The invention relates generally to apparatus and a method for using low-power homodyne continuous wave (CW) Doppler radar to mesure the size distribution of hydrometeors, e.g., falling raindrops and hailstones.
2. Background of the Invention:
The knowledge of hydrometeor dynamics, including data on the size, size distribution and fall speed of hydrometeors constituting observed precipitation is essential to an understanding and interpretation of atmospheric processes. This kind of detailed data of actual precipitation serves different purposes. Radar meterologists in both the experimental and applications areas rely on the determination of a drop size distribution curve to calibrate their radars. Without knowledge of the drop size distribution within the radar sample area the accuracies of radar derived rainfall rates are in serious doubt. The distrometer is a necessary tool used in the discipline of cloud physics. Hydrometeor size, number, and occurrence rate provide information essential to the evaluation of cloud physics experiments.
Millions of dollars are spent annually in the field of hail research. A basic requirement of the researcher is knowledge of the size of the hail and the relative number of hailstones falling per event. A third parameter that the research would have to have recorded is hail intensity and size as functions of time, to the nearest minute. Current instrumentation techniques do not provide this fine grain time data.
The relevant prior art, therefore, is that which involves apparatus and methods to provide a user with detailed data on the velocity and size distributions of liquid or solid hydrometeors, even from relatively inaccessible regions, in real time or near-real time.
Numerous instruments, of varying capabilities, have been developed to study the size distribution of falling hailstones. See Towery, N. C., S. A. Changnon, Jr. and G. M. Morgan, Jr., 1976: A Review of Hail Measuring Instrument, Bull. Amer. Metero. Sco., 57 1132-1140; and Nicholas, T.R., 1977: A Review of Surface Hail Measurement. Hail: A Review of Hail Science and Hail Suppression, Meteor. Monogr., 16, No. 38, 257, 267.
Among the available instruments is the Hailpad. Typical forms of this device are the Styrofoam Pads Manufactured by Dow Chemical Company and cut to size by the user. The hailpad is in its simplest form is a piece of flat inexpensive styrofoam material which is used to infer the size of falling hailstones by the depth and diameter of the impact craters left in a hailpad surface exposed to the hailstone precipitation. Hailstone sizes as small as 5 mm can be so measured and the rate of precipitation can be computed by also noting the hailpad exposure time.
Although the hailpad itself is a low-cost item, there are many problems in using it. No generally accepted calibration standards exist, human reading of each hailpad's indentations is tedious and time-consuming, only solid hyrometeors can be studied, wind may cause angled impacts requiring more detailed measurements of elongated craters, craters within or on earlier craters may cause errors, and styrofoam exposed to sunlight will deteriorate.
Another popular device, usuable with liquid hydrometeors, is the Joss-Waldvogel Distrometer. This device provides a record of the number and sizes of raindrops striking a diaphragm surface as functions of time. Typical of this device is the Distromet Ltd. system, comprised of model RD-69 sampler, Model AD-69 analyzer, Model Qt-30 timer and Interface unit FAD-3. Raindrop diameters in the 0.5 mm to 5.0 mm range are inferred from the diaphragm's displacement due to actual impacts.
The Joss-Waldvogel distrometer is sensitive to the effect of wind on the shape and alignment of droplets as they impact the sampler surface; is sensitive to noise and vibration, e.g., high sound levels, and is limited to a maximum droplet diameter of 5 mm for accurate measurements.
An instrument suitable for use with both solid and liquid hydrometeors is the Ground-Based Optical Array Precipitation Spectrometer manufactured by Particle Measurement Systems, Boulder, Colo. This is a relatively new instrument that will accurately measure the sizes of rain droplets with diameters in the 0.2 mm to 12.4 mm diameter range by means of an optical system to project droplet images on a 64 element photodiode array. Although expensive and fragile, the optical array spectrometer can be used for field measurements. It has a typical weight of 50 lbs. and a power requirement of 100 watts, which tends to limit its operational flexibility for long-term use in unattended remote locations.
A recent design for a digital distrometer employs a sampling head comprised of equally spaced wires. See Donnelly, D. P. and Bulson, T. M., 1980: Digital Distrometer. 19th Conference on Radar Meteorology, American Meterological Society, 486-489. The droplet diameter is assumed proportional to the number of wires encountered during each droplet's passage through the plane array of parallel wires. This technique cannot readily cope with a mixture of rain and hail or a precipitation of hailstones alone.
A paper titled: "A New Precipitation Occurrence Sensor System," by Sheppard, B. E., et al. was published in the Preceedings of "the Fifth Symposium on Meterological Observations and Instrumentation" sponsored by the Amercian Meterological Society, Toronto Canada, Apr. 15, 1983 pp 38-41. This paper discusses both apparatus and a method having some features similar to those of the present invention. The Sheppard, et al., reference discloses the use upwardly directed CW Doppler radar with 10.525 GHz Gunn diode, FFT processing of the Doppler spectrum, and a microprocessor to reduce the data further. The focus of this reference, however, appears to have been to note the start and stop time of the precipitation event, identification of the type of precipitation (as snow, rain or drizzle, or hail), and the precipitation amount, rather than the size distribution of the hydrometeors. There are, furthermore, numerous differences in the operation features of the circuitry between the sheppard et al. system and the present invention.
Scientific studies dating back to 1904 provide both carefully measured data on the terminal velocities of falling droplets and various curve-fits relating terminal velocities to hydrometeor diameters in both the liquid and frozen (or solid) state. Studies of particular interest include the following:
(i) Lenard, P., 1904: Uber Regen. Meteor. Z., 21, 248-262. Determined the terminal velocity of droplets in the weight range from 1,000 to 130,000 micrograms.
(ii) Gunn, R. and Kinzer, G., 1949: The Terminal Velocity of Fall for Water Drop in Stagnant Air. Journal of Meteorology 6, 243-248. One of the classical references, relating terminal velocity to drop diameter for droplets whose weights ranged from 0.2 to 100,000 micrograms.
(iii) Laws, J. O., and Parsons, D. A., 1943: the Relation of Raindrop Size to Intensity. Transactions of the American Geophysical Union, Papers on Hydrology, 452-459. Reviews the milestone measurements made before 1943, and includes their own measurements of drop size distribution as a function of several rainfall rates.
(iv) Marshall, J. S., and Plamer, W. McK., 2948: The Distribution of Raindrops With Size, Journal of Meteorology, 5, 165-166. Confirmed the earlier data of Laws and Parsons (1943) and the curve fits therefrom. Also improved coefficient values for the radar rainfall equation.
(v) Best, A. C., 1950: Empirical Formulae for the Terminal Velocity of Water Drops Falling Through the Atmosphere. Quarterly Journal of the Royal Metorological Society, 76, 302-311.
(vi) Atals, D., Srivastava, R. C., and Sekhon, R. S., 1973: Doppler Radar Characteristics of Precipitation at Vertical Incidence. Previews of Geophysics and Space Physics, Vol. 11, No. 1, 1-35. A comprehensive review and extension of the theoretical bases for measurement of the characteristics of rain and snow with vertically pointing Doppler radar.
(vii) Martner, B. E. and Battan, L. J., 1976: Calculations of Doppler Radar Velocity Spectrum Parameters for a Mixture of Rain and Hail. Journal of Applied Meteorology, 15, 491-498.
These studies, together with experimental data obtained from the hailpad and the Joss-Waldvogel distrometer, serve as sources of calibration data for the present invention, which provides a single, lowcost, relatively simple apparatus and method for the measurement of hydrometeor size distributions, in both the liquid and solid states, over a much wider range than was possible before.